In a conventional loudspeaker system, it is difficult to realize, due to an effect of acoustic stiffness caused by an internal cavity of a cabinet, a loudspeaker system which is small and capable of bass reproduction. This reproduction limit of bass is determined depending on a degree of the acoustic stiffness, that is, a capacity of the cabinet. Thus, as one of the solutions to the problem of the reproduction limit of the bass, a loudspeaker system having an aggregate of granular activated carbon located in an inside of the cabinet thereof is suggested (for example, see patent document 1).
FIG. 22 is a tectonic profile of a main section of a conventional loudspeaker system. In FIG. 22, the conventional loudspeaker system comprises a cabinet 90, a woofer 91, activated carbon 92, a supporting material 93, and a diaphragm 94. The woofer 91 is attached on a front surface of the cabinet 90. The activated carbon 92 is formed by granular activated carbon (hereinafter referred to as granular activated carbon), and located, in an aggregated state, in an inside of the cabinet 90. Further, the activated carbon 92 is supported by a back surface, a bottom surface, an upper surface, and right and left side surfaces of the cabinet 90, and the supporting material 93. Note that the supporting material 93 has pores, which allow passage of air, formed on an entire surface thereof.
Next, an operation of the conventional loudspeaker system shown in FIG. 22 will be described. When an electric signal is applied to the woofer 91, a sound pressure is generated. With the sound pressure, a pressure of the inside of the cabinet 90 changes. And with this pressure change, the diaphragm 94 vibrates. With this vibration of the diaphragm 94, a pressure of a cavity having the activated carbon 92 located therein changes. The activated carbon 92 is supported, in an aggregated state, by the supporting material 93 and the cabinet 90. Note that the entire surface of the supporting material 93 has, as above described, pores formed thereon. Therefore, with the pressure change caused by the vibration of the diaphragm 94, a gaseous body is physically adsorbed into the activated carbon 92, and the pressure change of the inside of the cabinet 90 is suppressed. That is, the cabinet 90 operates equivalently as a cabinet having a large capacity. In this way, in the conventional loudspeaker system, the activated carbon 92 equivalently expands an internal capacity of the cabinet, thereby enabling the bass reproduction, in spite of a small cabinet, as if the speaker unit is accommodated in a large cabinet.
Patent Document 1: Japanese National Phase PCT Laid-Open Publication No. 60-500645
Here, a structure of the activated carbon 92 will be studied. The activated carbon 92 of the above-described conventional loudspeaker system is formed by an aggregate of granular activated carbon having an average particle size of 0.1 mm to 0.3 mm. Countless numbers of pores are formed in an inside of the granular activated carbon. A specific surface area per unit weight of the granular activated carbon is about 1000 m2/g. As shown in FIG. 23, the pores are broadly classified into macro pores 100 formed in the vicinity of a surface of the granular activated carbon, and micro pores 101 formed in an inside thereof. FIG. 23 is a diagram typically showing a structure of the pore formed in the granular activated carbon. In FIG. 23, it is thought that the gaseous body is physically adsorbed into the infinite number of micro pores 101 formed in the inside of the granular activated carbon, whereby the activated carbon 92 exerts an effect of a capacity expansion as above described. Note that, each of the macro pores 100 functions as a path for the gaseous body to reach each of the micro pores 101.
However, in the granular activated carbon having a particle size no less than 0.1 mm, a volume ratio of macro pores 100 is larger compared to that of the micro pores 101, and thus an effect of physical adsorption is limited. Therefore, to obtain, significantly, the effect of the physical adsorption, a large amount of the granular activated carbon is required, and a volume of the activated carbon 92 is required to be expanded. However, in the case of a small-size loudspeaker system whose cabinet has a small internal capacity, the volume of the activated carbon 92 to be accommodated is limited. Therefore, the effect of the physical adsorption cannot be achieved sufficiently, and consequently it is difficult to expand a desired bass reproduction range.
Further, the macro pores 100, which function as the path for the gaseous body, act as acoustic resistance which suppresses a flow of the gaseous body reaching the micro pores 101. Therefore, there is a problem where a loss in acoustic energy is caused by the acoustic resistance, and consequently a sound pressure level of a bass range deteriorates significantly.
Further, when a frequency range of the sound pressure becomes high, the macro pores 100 form, in cavity capacities of the paths and lengths of the paths thereof, a high-cut filter which blocks off sound transmission. Accordingly, with respect to a high range of 100-200 Hz or higher, the flow of the gaseous body into the micro pores 101 is suppressed. Therefore, with respect to the high range of 100-200 Hz or higher, the effect of the physical adsorption is hardly obtained, and consequently there remains a big problem where use of the activated carbon 92 is limited to a loudspeaker system dedicated to a bass range of 100 Hz or lower.
Therefore, the present invention is related to a loudspeaker system to solve the above-described problem. In particular, the system is directed to improve expansion of an equivalent capacity by the effect of the physical adsorption by the activated carbon, and also prevent deterioration in the sound pressure level, which is caused by the loss in the acoustic energy, thereby realizing a small size loudspeaker system capable of reproducing rich bass.